The invention relates to load handling by a gripper, and particularly to load handling by gripping means that attach to one or more load fastening points.
Today, a vast majority of international sea freight is transported in containers. Such containers are boxlike transport units having standard dimensions, either 20, 40 or 45 feet in length. A container is about 2.5 m in width, and most typical container heights are about 2.6 m and 2.9 m.
The containers are provided with standardized corner castings enabling a container to be hoisted and carried by different container hoists. A container hoist is typically provided with a spreader suspended on hoisting ropes or chains, the spreader being adjusted by a telescopic mechanism according to the length of a container to be picked up, e.g. to a length of 20 or 40 feet. The corners of a spreader are provided with special turnable twist-locks enabling a container to be gripped. The corner castings of the container are provided with standardly shaped holes in which the twist-locks of the spreader are fitted. When the container hoist lowers the spreader on top of a container such that all four twist-locks of the spreader are received in the holes of the corner castings, the twist-locks may subsequently be turned by 90 degrees, making the twist-locks lock into the corner castings. The container may now be lifted into the air, suspending from the spreader.
The containers may be stacked on top of one another, typically e.g. five containers on top of each other. This enables a large number of containers to be stored within a small ground area, e.g. in a container port. The stacking of the containers has to be carried out carefully, such that the corner castings in the bottom of a container to be stacked are aligned with the corner castings provided in the roof of a lower container with an accuracy of at least about 5 cm. Otherwise there is a risk of the container stack collapsing.
A typical container hoist used for picking up and stacking containers is called a gantry crane, FIG. 1, which may move either on rails (Rail Mounted Gantry Crane or RMG) or on rubber tyres (Rubber Tyred Gantry Crane or RTG). When using a gantry crane, the containers (1) to be stored are placed in rows (a, b, c, d, e) between the legs of the gantry crane (14) such that typically 6 to 8 rows of containers are provided side by side between the legs of the gantry crane, the rows typically comprising e.g. five containers stacked on top of one another. A gap of typically 30 to 50 cm is then left between the containers and the container rows in order to make the containers easier to handle. Typically, a driveway (f) is left between the legs of the gantry crane to enable containers to be driven underneath the gantry crane to be stacked into rows (a, b, c, d, e).
In the gantry crane, the spreader (2) is by means of hosting ropes (16) suspended on a special trolley (15), which may be driven in a sideways direction of the container hoist into different positions, thus enabling containers in different rows to be handled. The length of the container rows may be up to hundreds of meters, i.e. several dozens of 20 or 40 feet containers in succession. A ship-to-shore crane is particularly designed for lifting containers to and from a ship. In such a case, the trolley of the crane is also capable of moving along a cantilever-like bridge so as to reside on top of a ship being handled.
Another typical container hoist is called a straddle carrier, which moves on rubber tyres and is considerably narrower than the gantry crane. The straddle container may accommodate only one row of containers, having typically 3 to 4 containers stacked on top of one another, between its legs. Now, a considerably larger gap, up to 1.5 m, is left between the container rows in order to provide enough space for the tyres of the straddle carrier to move between the container rows.
The spreader (2) of the container hoist is often also provided with a special fine transfer mechanism, in which case e.g. by means of stay ropes (17) of the suspension of the spreader or by means of different hydraulic cylinders it is possible to control the horizontal position and skew of the spreader without having to drive or move the trolley (15) or the container hoist (14) in its entirety. This property aims at making the containers easier and faster to pick up and stack on top of one another.
If the fine transfer mechanism of the spreader is implemented e.g. by stay ropes (17), the number thereof typically being four, i.e. one for every corner the container (FIG. 7), a desired force or difference of forces is then e.g. by electric motors generated in the stay ropes so as to move the spreader (2) horizontally (x, y) in a desired direction or to skew the spreader in a desired direction. The fine transfer mechanism of the straddle carrier is usually implemented by different hydraulic cylinders.
When a container handling machine, such as the aforementioned container hoists, is used for handling containers, high accuracy is required of a driver in several work phases. Examples of such work phases comprise gripping a container and stacking a container on top of another container.
In order to pick up a container, the spreader has to be controlled horizontally so as to accurately reside on top of the container to be picked up, either by using a fine transfer mechanism or by moving the trolley or the container hoist in its entirety such that when the spreader is lowered on top of the container, all four twist-locks of the spreader are accurately received in the holes of the corner castings of the container. The work phase is laborious for the driver of the container handling machine. In some cases, the picking up process is facilitated by mechanical guides mounted in the spreader. However, the guides make working in the narrow gaps between the containers more difficult.
In order to stack a container on top of another container, the spreader and the container suspending therefrom have to be accurately controlled so as to reside on top of a lower container, either by using a fine transfer mechanism or by moving the trolley or the container hoist in its entirety such that when an upper container is lowered all the way to the lower container, the corner castings in the bottom of the upper container align with the corner castings of the lower container as accurately as possible. A stacking accuracy to strive at is typically about 3 cm. As is understood by one skilled in the art, this work phase requires more time and accuracy of the driver of the container handling machine than picking up a container since now the alignment of the container on top of a lower container cannot be facilitated e.g. by simple mechanical guides, such as those mentioned above.
The operation of container hoists is becoming more and more automated such that the work of the driver of a container hoist is made faster and easier by means of computer control, for instance. When automation is taken further, it is possible to even eliminate the driver from the container handling machine, in which case the container handling machine operates either by remote control and/or completely independently, controlled by a computer. It is often also possible to perform work phases in a flexible manner such that if in some case an automatic work phase fails, a remote-control operator may then carry out the work phase exceptionally by remote control. For instance in such a situation it would be advantageous that the technology used could flexibly adapt to be both a function assisting the driver and an automatic function controlled by a computer.
A previously known solution to assist the driver in gripping a container utilizes Charge-Coupled Device or CCD cameras installed in a spreader and oriented downwards for transmitting video footage enabling the driver to align the spreader on top of a container. However, the method is not suitable for automating the operation since image processing algorithms that could enable the location of a container to be monitored from camera footage function unreliably in different weather and lighting conditions. The method is not suitable for stacking containers, either, since while stacking containers, a camera sensor is located quite far away from a lower container (3 m) and, in addition, an upper container blocks the driver's view either for the most part or completely. Furthermore, the upper container casts a shadow on the lower container, making the gap between the containers very dark. In such a case, it is very difficult to discern the lower container in the video footage.